The present invention relates to a spray-coating device, and more particularly to a controlled oscillator for driving the transformer of a spray-coating device wherein the oscillator frequency is constantly and automatically adjusted to oscillate at the resonant frequency of the transformer.
A spray-coating device is described in U.S. Pat. No. 3,895,262. In the patent, an oscillator produces an alternating voltage for driving a step-up transformer. The output of the transformer is converted by a voltage multiplier circuit to a very high DC voltage for a charging electrode that is deployed for charging powdered or liquid coating material. The voltage multiplier circuit can be a cascade circuit formed by rectifiers and capacitors or by an arrangement of resistors.
Optimally, in order to reduce heating of the transformer, the oscillator should be operated at a particular frequency, namely the resonant frequency of the transformer. That resonant frequency is determined by the inductance of the transformer and its capacitance. The latter arises from and is determined by the number of turns on the secondary winding of the transformer. When the transformer is operated outside its resonant frequency, the transformer current--and hence the loss in the transformer due to transformer heating--increases sharply.
Ordinary oscillators have inherent disadvantages. For example, their outputs are not precisely sinusoidal. Moreover, conventional oscillator outputs contain undesired harmonics and the electrical power at the outputs tends to vary in response to different factors as, for example, the type of signal being produced. This results in higher reactive powers in a transformer driven by a conventional oscillator and consequently to relatively strong heating of the transformer. The problem of overheating is particularly acute in spray-coating devices which incorporate the transformer within the spraying device, as for example in a hand held spraying gun.
Conventional oscillators have other disadvantages as well. For example, they typically oscillate only while actually connected to a transformer because they depend on the inductance and capacitance (LC) of the transformer and of the spraying gun to oscillate. The oscillator frequency is, therefore, affected in part by the transformer. This makes for more difficult control of the output frequency of conventional oscillators.
Devices, other than oscillators, are known in other arts for producing alternating voltages. For example, Federal Republic of Germany OS 34 31 001 A1 describes a device with an induction coil for heating a material by operating the induction coil at its resonant frequency. However, because the type and the size of the material being heated and its proximity to the induction coil affect the inductance of the coil, a phase shift correcting circuit is necessary for regulating the phase shift to reduce reactive power.
Federal Republic of Germany OS 36 01 191 A1 similarly discloses a phase-regulating device for a parallel resonant circuit converter. Also, general-purpose digital sine-wave generators are described in Federal Republic of Germany OS 20 45 971 and in the journal "Elektronik" 5/Mar. 11, 1983, pages 53 to 57.